Note: This glossary contains technical terms and is designed for persons with a scientific background.
The name given to volcanic rocks that have chemical compositions intermediate between basalt and rhyolite. The mineral assemblage is typically dominated by plagioclase plus pyroxene and/or hornblende. This is the type of magma/lava erupted at the Soufrière Hills Volcano in Montserrat.
This is defined as volcanic particles of pulverized rock and glass less than 2 mm in diameter.
These are large fragments of lava and/or hot rock forcibly ejected by volcanic explosions which are typically about 0.5 to 2.0 m in diameter.
The type of magma thought to be entering the magma reservoir beneath the Soufrière Hills Volcano, chemically “less evolved” than andesite. It is usually gray to black and fine-grained due to rapid cooling of lava at the surface of a planet.
A large depression generated by the collapse of a volcanic edifice into its magma chamber. Transfer of magma from the chamber to the surface causes the edifice to founder (sink below the surface), either as a coherent piston or chaotically as a jumble of blocks. Some calderas form due to prolonged eruption of basaltic lava, like on Hawaii. The world’s largest calderas form during explosive eruptions and can be several tens of kilometres wide and up to two or more km deep. For example, the great caldera of Lake Toba in Sumatra is 100x30km. It formed by collapse associated with more than one ignimbrite eruption, the last of which was about 75,000 years ago. Some small calderas are only a few km across. The composite caldera of Santorini is 6x8km and extends to 400m below sea level. It formed as a result of at least four explosive eruptions during the last 200,000 years.
Magma flows to the Earth’s surface via a pathway known as a “conduit”: a channel created inside a volcano by molten magma and by which magma travels to the surface. The exact size and shape of the conduit beneath the Soufrière Hills Volcano is not known.
This means Correlation Spectrometer and is the instrument that measures the amount of ultraviolet light absorbed by sulfur dioxide molecules within a volcanic gas.
Cristobalite A silicate mineral that is a high-temperature form of quartz; stable above 1470°C; crystallizes in the tetragonal system at low temperatures and the isometric system at high temperatures. It forms by precipitation of tiny crystals in the pores of the Soufrière Hills andesite and by devitrification of the volcanic glass.
This is one of the principal phenomena monitored during an eruption. The surface of the volcano responds to changes within the interior of the volcano or deeper in the magma chamber. The volcano can swell (inflate) or subside (deflate) to allow inferences to be made about the magma pressure.
Differential Optical Absorption Spectroscopy. DOAS instrumentation operates on a well-established scientific principle, Beer-Lambert’s absorption law, which relates the quantity of light absorbed to the number of gas molecules in the light path. At the MVO it is used in the processing of the UV spectral data, whereby the “blue” or background atmospheric spectrum is subtracted from the spectra of the emissions plume – allowing calculation of SO2 emission rates from the volcano.
E – F
Fourier Transform Infra-Red spectroscopy. A method used to measure variations in the ratio of hydrogen chloride and sulphur dioxide gases emitted by the volcano. It utilizes the differing light absorptions in the infra-red region of the electromagnetic spectrum.
An opening in Earth’s crust, often in the neighbourhood of volcanoes, which emits steam and gases such as carbon dioxide, sulfur dioxide, hydrochloric acid, and hydrogen sulfide. It is often accompanied by sulphur deposition around the opening of the vent.
On slow cooling, the liquid portion of the magma usually crystallizes, but if cooling is sufficiently rapid, it may convert to “glass”. This commonly occurs when hot magma is “chilled” as it mixes with air or water.
Volcanic gases are dissolved in the magma at depth in the chamber and are released in the low-pressure environment of the earth’s surface. The main volcanic gas is usually water with minor amounts of sulphur dioxide, carbon dioxide and halogen gases such as chlorine and fluorine.
This stands for Global Positioning System. It is a navigation system which allows the location of a point on the earth’s surface to be determined precisely. This is done when a receiver obtains signals from satellites which allow the location to be fixed. MVO uses GPS to monitor long term deformation – movement of fixed points on the volcano’s flanks.
A complex hydrous silicate of calcium, magnesium, and iron which appears green to black in colour. It is formed in the late stages of cooling in igneous rock and is observed as crystals in the lavas of Montserrat.
A hybrid earthquake is characterized by seismic signals containing long and short frequencies. This type of earthquake occurs at shallow depth (usually less than 2 km) and is interpreted as fractures forming under high gas pressures. Hybrid earthquakes are often indicative of magma motion in the upper part of the conduit or the dome. They have a mixture of the characteristics of volcano-tectonic and long-period earthquakes.
This describes the swelling of a volcano due to an increase in internal pressure.
J – K – L
Lahar is an Indonesian word for a rapidly flowing mixture of rock debris and water that originates on the slopes of a volcano. Lahars are also referred to as volcanic mudflows or debris flows. They form during and after heavy rainfall on the upper slopes of the volcano where loose deposits of volcanic ash and fragments of dome-rock lie.
Powerful volcanic explosions with a significant directed horizontal component (the lateral blast in the strict sense), which can generate devastating, high-energy pyroclastic flows and surges.
Once the magma reaches the Earth’s surface and is extruded in the form of molten rock, it is known as “lava”.
These are morphologies formed by eruption of extremely viscous or semi-solid magma which piles up around and above the vent. The lava is too viscous or stiff to flow away from the vent and a thick pile forms.
LP [Long-Period earthquake]
These have low frequencies and are thought to be caused by the movement of magma in the upper conduit causing resonance.
This is molten or semi-molten rock that has sufficiently melted to be able to flow to the earth’s surface and erupt. The ‘magma chamber’ is the region which supplies magma to the volcano.
A large underground pool of molten rock (magma) lying underneath the volcano.
The mineral form of black iron oxide, Fe3O4, that often occurs naturally with magnesium, zinc, and manganese and is an important ore of iron.
A naturally occurring solid formed through geological processes that has a characteristic chemical composition, a highly ordered atomic structure, and specific physical properties. Magmas crystallize different minerals as they cool from high temperature. Common minerals in rocks include olivine, pyroxene, plagioclase, magnetite, and hornblende. Many types of lava consist of millimetre-sized phenocrysts of different minerals set in a fine groundmass, the crystals of which are only visible under a microscope.
Also known as lahars. They are concentrated slurries of volcanic debris and water. They form during and after heavy rainfall on the upper slopes of the volcano where loose deposits of volcanic ash and fragments of dome-rock lie. The term mudflow is slightly misleading given the water content can vary dramatically as can the materials carried, from very fine ash to house-sized dome rock boulders!
N – O – P
Crystals in a lava, pumice, or scoria which form by slow cooling of the magma in the magma chamber. When the magma rises to the surface it cools more rapidly and the remaining melt either chills to a glass (as in pumice or scoria) or crystallizes to a fine mesh of microscopic crystal called the groundmass (as in many lavas). Phenocrysts in Montserrat rocks may reach a millimetre or more in size, and may sometimes occur in clusters.
Phreatic explosions are caused by ground water being heated by rising magma. The phase change from superheated liquid to vapour (steam) close to the earth’s surface causes explosive activity.
A mineral with the chemical composition calcium sodium silicate formed by the crystallization of all magmas across the range basalt to rhyodacite.
A sustained, explosive eruption which forms a high, jet-like column of pumice and ash in the atmosphere. As silica-rich magma such as dacite or rhyodacite rises from depth, dissolved gases come out of solution and form bubbles. When the percentage of bubbles reaches about 50%, the magma froth blows itself apart, forming a mixture of pumice, ash, and rock fragments dispersed in gas. This mixture then accelerates to the surface and erupts at supersonic speeds. Plinian eruptions commonly last several hours and lay down thick layers of pumice as pyroclastic fall deposit. The clouds of such eruptions may be 30 km or more high and reach well into the stratosphere. Named after the AD79 eruption of Mount Vesuvius, Italy, described by Pliny the Younger.
A porous rock formed during explosive eruptions, generally of silica-rich magma. Gas dissolved in magma at high pressure comes out of solution as the magma ascends towards the earth’s surface. This forms a froth which then fragments violently, driving the explosive eruption. Pumice clasts are pieces of the magmatic froth chilled to glass in contact with the air. Pumice commonly contains phenocrysts of plagioclase, hornblende, and magnetite. About 80% of many pumices are void space, causing them to float on water.
The word pyroclast is derived from the Greek πῦρ, meaning fire, and κλαστός, meaning broken. This is a general term for volcanic material composed chiefly of rock fragments of explosive origin, especially those associated with explosive volcanic eruptions.
A highly mobile avalanche [flow] of high-temperature volcanic (pyroclastic) debris (lava blocks and volcanic ash) and superheated volcanic gases and air that moves rapidly in response to gravity. Pyroclastic flows are common products of eruptions at Montserrat and elsewhere. Pyroclastic flows can form in several different ways: or as the result of gravitational collapse or explosive disruption of a lava dome and by explosive eruptions when an eruption column collapses – when they often additionally contain pumice blocks. Velocities as high as 60 m/s have been measured on historic pyroclastic flows, and some are believed to travel as fast as 250 m/s. Temperatures can exceed 1000 degrees Celsius. Owing to their high velocities and to fluidization by escaping gases, some pyroclastic flows may travel many tens of kilometres from the source vent, and may even travel large distances across the sea, as well documented at Krakatau in 1883.
Pyroclastic flows which contain abundant pumice give rise to the deposits often called ignimbrites.